Is Salaat valid with jeans and T-shirt?

Answer

A man who dresses in this lewd kuffaar style is a faasiq. He is like a naked person. The Salaat of a naked person is not valid. This type of fussaaq/kuffaar dress explicitly reveals the shape of the man's satr. No amount of technical skulduggery and humbuggery can validate Salaat performed with such disgraceful and haraam kuffaar dress style.

The essential requisite for even the technical validity of Salaat is proper and valid covering of the satr area of the body. The tightfitting pants is nugatory of valid concealment of the satr, hence the Salaat is not valid.

Assuming that someone conceals his satr adequately with kuffaar/mushrik flowing robes such as the garments of pundits, priests and the like, then too the Salaat will not be valid if proper Islamic garb is available. In fact, voluntarily donning religious garb of the kuffaar is by itself kufr.